In many technical fields, in particular in the field of microlithography, it is desirable, inter alia, to subject bodies and the surfaces thereof to a detailed optical inspection in order, for example, to be able to assess the quality of a production process and, where desired, to be able to intervene in a correcting manner to the extent that the inspection determines that predetermined quality criteria are not satisfied. Naturally, the same, if not even higher desired performance properties are to be placed in this case on the precision of the imaging device used for the inspection in comparison with the devices used in the production process of the body to be inspected.
In this context, the capability of the imaging device, used for the inspection, of being able to process light with different wavelengths with aberrations that are as small as possible is of particular importance in order to ensure a broad field of application of the imaging device. Therefore, it is desirable or advantageous, in particular in the context of production methods which include an optical process, if the imaging device used for the inspection is able to process with minimized aberrations the wavelength range which is also used during the optical process. By way of example, this relates to the wavelength range from 193 nm (so-called VUV range) to 436 nm (so-called Hg g-line).
Only few optical materials have a sufficient transparency in this wavelength range, and so the systems are predominantly constructed from synthetic fused silica (SiO2) and fluorite (CaF2).
In this case, the chromatic aberrations, i.e. the aberrations dependent on the wavelength of the light, are problematic. By way of example, if an imaging device with refractive optical elements (such as lens elements or the like) is used for the inspection, the aberrations of the imaging device are generally only minimized with justifiable outlay for a comparatively narrow wavelength range. So-called achromatization of such a dioptric imaging device, i.e. an imaging device including only refractive optical elements, i.e. an elimination of such chromatic aberrations is hardly possible with justifiable outlay over a broadband wavelength range (such as the one specified above).
Therefore, use is often made of so-called catadioptric imaging devices, which, in addition to refractive optical elements, also include reflective optical elements, which are advantageous in view of chromatic aberrations. By way of example, such catadioptric systems are known from U.S. Pat. No. 5,031,976 (Shafer), U.S. Pat. No. 5,717,518 (Shafer et al.), U.S. Pat. No. 7,136,159 B2 (Tsai et al.) and US 2004/0027688 A1 (Lange), the complete disclosures of which are respectively incorporated herein by reference. Here, US 2004/0027688 A1 (Lange), inter alia, discloses a high aperture (numerical aperture NA greater than 0.90), strongly magnifying catadioptric microscope objective in the context of wafer inspection, to which a zoom system, i.e. an optical system with a variable magnification or focal length, is connected.
In general, optical systems with refractive elements can no longer be used economically for applications with a very broad wavelength range and/or very short wavelengths (typically less than 190 nm). In this case, such an optical system corrected over a broad bandwidth is typically constructed from mirrors only, i.e. it has a catoptric design, as it is known, for example, from U.S. Pat. No. 3,811,749 (Abel), the entire disclosure of which is included herein by reference.
If an optical system corrected over a broad bandwidth is intended to be realized with a zoom system, the zoom system should naturally also have a catoptric design in that case. Such a catoptric zoom system is known from, for example, U.S. Pat. No. 4,812,030 (Pinson), the entire disclosure of which is included herein by reference. Here, a single mirror is displaced in order to change the focal length of the overall system, wherein, however, the image plane is also correspondingly co-displaced. A similar effect can also be achieved by interchanging a mirror, as is known, for example, from U.S. Pat. No. 4,964,706 (Cook), the entire disclosure of which is included herein by reference. However, the image plane is displaced with the change in the overall focal length in this case too. However, such a displacement of the image plane is often undesirable, as it makes the overall system more complicated or more expensive.
Since the angles of incidence on the mirrors also change during zooming, the movement of three mirrors is generally advantageous for a sufficiently good imaging quality in the case of a fixed position of the image plane. Such a catoptric zoom system with a fixed position of the image plane is known from, for example, U.S. Pat. No. 5,144,476 (Kebo), the entire disclosure of which is included herein by reference. Here, three of the four mirrors are displaced in order to keep the image plane stationary. However, the position of the beam incident in the zoom system is not constant in this case, which makes the use of this zoom system as a partial system of a larger overall system more difficult.
Finally, in a generic zoom system, it is also possible to interchange a plurality of mirrors in order to realize different magnifications in the case of a fixed position of the image plane, as is known, for example, from U.S. Pat. No. 5,009,494 (Iossi et al.), the entire disclosure of which is included herein by reference. There, the entire optical unit of the zoom system consisting of three mirrors is interchanged in order to vary the magnification of the imaging. However, the position of the beam incident in the zoom system is, in turn, not constant here either, which also makes the use of this zoom system as a partial system of a larger overall system more difficult.